The present disclosure generally relates to subterranean cementing operations. More particularly, the present disclosure relates to cementing plugs in stage cementing operations and associated methods of use.
During the drilling and construction of subterranean wells, casing strings are generally introduced into the well bore. To stabilize the casing, a cement slurry is often pumped downwardly through the casing, and then upwardly into the annulus between the casing and the walls of the well bore. One concern in this process is that, prior to the introduction of the cement slurry into the casing, the casing generally contains a drilling or some other servicing fluid that may contaminate the cement slurry. To prevent this contamination, a subterranean plug, often referred to as a cementing plug or a “bottom” plug, may be placed into the casing ahead of the cement slurry as a boundary between the two. The plug may perform other functions as well, such as wiping fluid from the inner surface of the casing as it travels through the casing, which may further reduce the risk of contamination.
Similarly, after the desired quantity of cement slurry is placed into the casing, a displacement fluid is commonly used to force the cement into the desired location. To prevent contamination of the cement slurry by the displacement fluid, a “top” cementing plug may be introduced at the interface between the cement slurry and the displacement fluid. This top plug also wipes cement slurry from the inner surfaces of the casing as the displacement fluid is pumped downwardly into the casing. Sometimes a third subterranean plug may be used, to perform functions such as preliminarily calibrating the internal volume of the casing to determine the amount of displacement fluid required, for example, or to separate a second fluid ahead of the cement slurry (e.g., where a preceding plug may separate a drilling mud from a cement spacer fluid, the third plug may be used to separate the cement spacer fluid from the cement slurry), for instance.
In some circumstances, a pipe string will be placed within the well bore by a process comprising the attachment of the pipe string to a tool (often referred to as a “casing hanger and run-in tool” or a “work string”) which may be manipulated within the well bore to suspend the pipe string in a desired sub surface location. In addition to the pipe string, a sub-surface release cementing plug system comprising a plurality of cementing plugs may also be attached to the casing hanger and run-in tool. Such cementing plugs may be selectively released from the run-in tool at desired times during the cementing process. Additionally, a check valve, typically called a float valve, will be installed near the bottom of the pipe string. The float valve may permit the flow of fluids through the bottom of the pipe string into the annulus, but not the reverse. A cementing plug will not pass through the float valve.
When a first cementing plug (often called a “bottom plug”) is deployed from a sub-surface release cementing plug system and arrives at the float valve, fluid flow through the float valve is stopped. Continued pumping results in a pressure increase in the fluids in the pipe string, which indicates that the leading edge of the cement composition has reached the float valve. Operations personnel then increase the pump pressure to rupture a frangible device, within the bottom plug. Said frangible device may be in the form of a pressure sensitive disc, rupturable elastomeric diaphragm, or detachable plug (stopper) portion which may or may not remain contained within the bottom plug. After the frangible device has ruptured, the cement composition flows through the bottom plug, float valve and into the annulus. When the top plug contacts the bottom plug which had previously contacted the float valve, fluid flow is again interrupted, and the resulting pressure increase indicates that all of the cement composition has passed through the float valve.
Conventional cementing plugs are formed with wiper fins on their exterior surface, which function to wipe the pipe string as they travel downhole. Conventional cementing plugs used to wipe large diameter casing strings (18⅝ and larger) are by their very nature expensive to make, both heavy and bulky to handle, and require additional time to drill out due to the sheer volume of drillable materials to be removed. Under some conditions it may be advantageous to the well operator to run casing strings consisting of two or more pipe sizes, with the larger pipe size being at the shallowest depth and progressively tapering to the minimum pipe size. These casing configurations are typically known as “tapered strings” and require specially designed cementing plugs to wipe the different pipe diameters involved. Conventional cementing plugs are thus, fairly complex devices that are relatively time-consuming and as a result, expensive to manufacture, difficult to use, and are more costly to drill out due to the increased plug length and/or material content.
In addition, cementing plugs may be required to pass through internal restrictions designed into special tools which may be incorporated into the pipe string, such as the seats in a plug operated multiple stage cementing device. The specially designed cementing plugs required to pass through these types of internal restrictions must both effectively wipe the casing internal diameter and pass through the internal restrictions with minimal pressure increase to avoid prematurely activating the tool. In these instances, it is generally impossible to place the special devices in tapered strings unless the device is located in the largest pipe size due to the increased pressure that would otherwise be required to force the mass of the larger wiper segments through the restrictions.
With the increased sophistication of cementing operations, different types of fluids may need to be displaced through the casing. To prevent contamination and or intermixing of the fluids, multiple cementing plugs or bottom plugs may be advantageous. In these operations, plugs, particularly floppy wiper plugs or darts may be used. As such, there is an increased risk of plugs wedging beside each other, which may increase circulation pressures significantly or could potentially bridge the casing against further fluid displacement.